1. Field of the Invention
This invention relates to the thermal treatment of glass, and more especially to the thermal toughening of glass articles, for example flat glass or bent glass sheets. Such thermally toughened glass sheets may be for use singly as a motor vehicle windscreen, or as part of a laminated motor vehicle windscreen, a side light or rear light for a motor vehicle, or for use in the construction of windscreen assemblies for aircraft and railway locomotives, or in the construction of windows for ships, or for architectural uses. Other glass articles such as pressed or blown glass articles may be thermally toughened by the method of the invention.
2. Description of the Prior Art
The ultimate tensile strength of a glass article can be increased by a thermal toughening process in which the glass is heated to a temperature approaching its softening point, followed by rapid chilling of the glass surfaces to induce centre-to-surface temperature gradients through the thickness of the glass. These temperature gradients are maintained as the glass is cooled through its strain point. This results in compressive stress in the surface layers of the glass sheet with compensating tensile stress in the central core of the thickness of the glass sheet.
Usually this thermal toughening process is carried out using chilling air directed uniformly at both surfaces of the glass sheet out it is difficult to obtain a high degree of toughening using air flows, particularly when toughening glass sheets of 3 mm thickness or less. Attempts to increase the degree of toughening of a glass sheet by increasing the rate of flow of cooling air can give rise to loss of optical quality of the surfaces of the glass and distortion of the shape of the glass sheet due to the buffeting action of the chilling air.
In another thermal toughening process a glass sheet at a temperature near to its softening point is quenched in a chilling liquid. High stresses can be produced by this method. The glass sheets have to be cleaned after quenching.
Thermal toughening of a glass sheet has also been proposed by a method in which a hot glass sheet is immersed in what, in practice, was a freely-bubbling fluidised bed of solid particles, for example sand.
Such a process has not been brought into commercial use hitherto.
The major problem which we have found when attempting to operate such a bed for the thermal toughening of glass is the high incidence of fracture of the glass sheets during their treatment in the fluidised bed. The fracture of a glass sheet while being quenched in a freely-bubbling fluidised bed is thought to be caused by the induction of destructive tensile stresses in the leading edge of the glass sheet due to nonuniform cooling as the leading edge enters the bed of particles in a state of bubbling or aggregative fluidisation.
Loss of glass sheets due to fracture is particularly serious when attempting to toughen thin sheets of glass, for example of thickness from 2.3 mm to 4.0 mm, to a high stress value, and has been such as to render the process unacceptable for the commercial production of toughened glass sheets for use in car windscreens for example. The problem of fracture also arises to a lesser but still commercially significant extent when seeking to toughen thicker sheets, for example up to 8 mm thick.
A freely-bubbling bed in a state of aggregative fluidisation has also found to damage hot glass sheets immersed in it. This is due to the irregular forces to which the glass is subjected in a freely-bubbling bed. This can give rise both to changes of overall shape and to more localised surface damage, the former occurring particularly with thinner glass sheets such as those of 2 mm to 3 mm thickness. Such damage as changes of shape may give rise to difficulties in lamination, and surface damage may give rise to unacceptable optical quality when the sheet is used as a window or as a component of a laminated window.
The present invention is based on the discovery that the use of a gas-fluidised bed in a quiescent uniformly expanded state of particulate fluidisation unexpectedly produces adequate stresses in glass sheets quenched in it and substantially reduces loss of glass sheets due to fracture in the bed or to change of shape or surface damage so that a successful commercial yield is achieved.